Sorption is an important process governing the fate of contaminants in groundwater . The proposed research will assess the effect of multi-solute mixtures on sorption of contaminants by soils. Specifically, the proposed research will investigate competition among solution components for sorptive sites. Competition will be assessed experimentally by equilibrium isotherm testing, and will be modeled using stoichiometric, thermodynamic, and empirical relationships. The project will focus initially on the sorption of two organic solutes from the hazardous substance list (viz., toluene and xylene) by chromatography-grade silica and alumina. It is anticipated that later studies may involve other solutes and also natural sorbents. In the absence of specific knowledge about competition, engineers who predict the mobility of contaminants in soils at Superfund and other contaminated sites must assume that competition does not occur, and therefore may overestimate the sorptive capacity of soils. As a result, the rate of contaminant migration may be underestimated, and thus the risks posed by off-site migration of contaminants in groundwater may also be underestimated. The results of the proposed research will enable engineers to account for competition. One distinguishing feature of this project relative to other past or ongoing efforts under the Superfund program is that it will focus on sorbents characteristic of aquifer materials, which are materials for which sorption is relatively weak and, therefore, for which sorptive transport is critical to the overall migration of contamination. Another distinguishing feature of the proposed research is that it will attempt to develop theoretical or empirical relationships which can be used to predict sorption of multisolute components based upon equilibrium isotherm and thermodynamic properties of the sorbate/sorbent system. These relationships could then be incorporated into existing materials balance models for multi-solute transport through porous media to enable estimation of the chromatographic effect of competition.